Lathes are mechanical devices used in the art of carving wood pieces, generally into bowls and other shapes. Lathes are also used for some applications for the moulding of metals. The lathes operate by spinning wood pieces at high velocity while the operator carves the piece using a sharp instrument. The usual lathe has a flat horizontal bed with flat horizontal support rails mounted upon the lathe bed. Vertical support members cam connect the support rails to the lathe bed. A motorized unit for rotating the wood piece normally extends vertically above the lathe bed.
When carving a wood piece it is necessary for the lathe operator to make use of a fixed and stable support bar on which the carving instrument is supported while carving the wood piece. Without this support bar it would be impossible for the operator to have control over the carving instrument while working on the object of manufacture.
The support bar is the top member of a toolrest bracket referred to in the art as a banjo. A banjo consists of a horizontal support which is secured onto the support rails of a lathe bed by way of a clamping mechanism. A vertical portion of the banjo extends upwardly at one end. A top horizontal support bar which is rotatably disposed on the vertical portion of the banjo constitutes the actual toolrest.
Current mechanisms for clamping a banjo to support rails provide for poor or difficult clamping. These mechanisms typically consist of a bolt which extends through a slot where the bolt and the slot are both movable by the action of a handle which is attached to the bolt and is used to rotate the bolt. Two longitudinal clamps which are closed by the turning of the bolt are used to clamp the banjo to the support rails. A handle may be attached to the bolt either above or below the support rails. In either case, the positioning is awkward for the carpenter and the clamping effectiveness is often unsatisfactory.
Another design which is currently in use makes use of a cam in order to effect the clamping action necessary to clamp the banjo to the support rails. This design makes use of an elongate drive shaft which is supported in the banjo. The shaft has cams and support bearings at each end of the shaft. A lever is attached to the shaft to rotate the cams from an unclamped to a clamped position. In this case the rotation of the cams transfer upward movement to the shaft which is in turn transferred to an eye bolt which is moved vertically to pull up on a clamp plate which clamps to the support rails.
This design is problematic in that the drive shaft is subject to a downward pull during the clamping attempt due to the fact that the cams are located on the ends of the drive shaft. As such, when the clamping is to be effected towards the centre of the drive shaft, the drive shaft is prone to bend or deflect. As the drive shaft length is increased the degree of bending increases proportionally and leads to greater deflection.